Curable fluorosilicone rubber compositions are described in commonly assigned Jeram, U.S. Pat. No. 4,029,629 and 4,041,010. They are fluids, typically with viscosities in the range of 1,000 to 200,000 centipoise at 25.degree. C., using a vinyl terminated base polymer with a fluorosilicone content within the range of 5 to 98 mole percent. The compositions cure by means of a platinum catalyzed hydrosilylation reaction, and organic hydroperoxides, alkenyl-containing cyclic polysiloxanes and the like, serve as cure inhibitors.
The vinyl terminated base polymers have been disclosed to have the general formula: ##STR1## wherein Vi is vinyl, R is selected from alkyl of from 1 to 8 carbon atoms or phenyl, R.sup.1 is perfluoroalkyl of from 1 to 8 carbon atoms, x and y are at least 1 and, as mentioned, the concentration of the siloxy unit taken y times varies from 5 to 98 mole percent.
The vinyl terminated base polymer is prepared in large scale by the copolymerization of methyl-3,3,3-trifluoropropylsiloxane cyclic trimer and octamethylcyclotetrasiloxane using an alkali metal hydroxide, preferably potassium hydroxide. Polymerization is of an equilibration type, although it may also use complex cation catalysts, see, e.g., Evans U.S. Pat. No. 4,122,247 and 4,157,337. The vinyl end groups are established through the use of vinyl chain stoppers such as CH.sub.2 .dbd.CHSi(CH.sub.3).sub.2 O--Si(CH.sub.3).sub.2 -O--.sub.20 Si(CH.sub.3).sub.2 CH.dbd.CH.sub.2 or CH.sub.2 .dbd.CHSi(CH.sub.3).sub.2 --O Si(CH.sub.3).sub.2 -O-].sub.38 Si(CH.sub.3)(CF.sub.3 CH.sub.2 CH.sub.2)-O--.sub.23 Si(CH.sub.3).sub.2 CH.dbd.CH.sub.2, and the like, in known ways, and the amount of chain stopper controls the desired viscosity, also as is well known.
Although when properly compounded and cured, the prior art vinyl terminated based polymers provide good mechanical properties, they do not meet all of the requirements specified for the manufacture of the highest quality electrical connectors.
It has now been discovered that the prior art base polymers contain a small, unexpected amount of unsaturation on the polymer backbone, in addition to that to be expected, i.e., on the ends. It has been further discovered that such backbone unsaturation is detrimental toward the development of optimum mechanical properties, in that it leads to a tightly cured matrix which, in turn, adversely affects tensile strength, tear resistance and resilience.
Backbone unsaturation arises because fluorosilicone cyclic trimer used to make the base polymer in the above-mentioned equilibration process has been found to contain, or to produce by a side reaction in the presence of alkali metal hydroxide at elevated temperatures, a vinyl containing trimer, having the formula: ##STR2## Typically, the amount of such material in the cyclic trimer is never less than 1.5% by weight. It has now been found that using less than conventional amounts of base catalyst, and lower than conventional temperatures, can reduce the vinyl byproduct content--3,3-difluoropropenyl type unsaturation--to less than 1, and even down to 0 to 0.4 weight percent. If the low vinyl trimer-containing product is then copolymerized with cyclic tetramer following conventional procedures, there will be obtained a new base polymer with little or no backbone unsaturation. This is uniquely suitable for providing solvent resistant fluorosilicone rubber containing compositions with the best combination of physical properties.